The present invention relates to a dry compressing vacuum pump having a continuous or graduated inner compression and comprising a gas ballast device.
The term xe2x80x9cdry compressing vacuum pump with inner compressionxe2x80x9d denotes any vacuum pump, the pump chamber or pump chambers of which is/are free of oil and where the volume of the pump chamber decreases in a continuous or graduated manner from the inlet to the outlet of the pump. An example for a dry compressing vacuum pump having a continuously decreasing pump chamber volume is a screw vacuum pump with threads, the pitch, depth and/or width of which decrease continuously from the inlet to the outlet. Examples for dry compressing vacuum pumps where the inner compression decreases in a graduated manner are multi-stage claws, Roots or piston vacuum pumps in which the volume of the pump or compression chambers decreases from stage to stage. Also in the instance of screw vacuum pumps it is known to design the threads such that these change their properties in a graduated manner.
Dry compressing vacuum pumps are generally employed in applications (semiconductor production, for example) in which toxic, very expensive or also explosive gases need to be pumped.
It is known to employ in the instance of dry compressing vacuum pumps of the kind mentioned, gas ballast devices for the purpose of avoiding condensation in the area on the outlet side. The gas ballast is therefore supplied into the pump chambers or pump chamber sections at the outlet area.
Dry compressing vacuum pumps of the kind affected here exhibit, owing to their inner compression in the area of their outlet, pressures which not only exceed the inlet pressure but which can also significantly exceed atmospheric pressure. This also applies to the instance in which bypass valves are employed, since these valves throttle a large gas flow owing to their limited cross sections. Would a vacuum pump of the kind affected here be operated during this operational phase with an open gas ballast valve, then gases pumped by the pump would enter from the pump chamber into the atmosphere.
It is the task of the present invention to design the gas ballast device for a vacuum pump of the kind mentioned above in such a manner that the risk of gases escaping no longer exists. Moreover, it shall be achieved that gas ballast operation will not impose an additional load on the pump""s drive motor.
This task is solved through the characterising features of the patent claims. In that a component of the gas ballast device is a non-return valve, it can be ensured that gases pumped by the pump can not escape to the outside through the gas ballast device.
It is expedient to provide, in addition, a differential pressure valve which allows the admittance of the ballast gas only starting at a certain pressure difference. Through this measure it can be ensured that the admission of the gas ballast into the vacuum pump will only be possible at a pressure below that defined through the differential pressure valve. Unnecessary loading of the pump by the admitted ballast gases can thus be avoided.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.